The purpose of this project is to study the physical properties of a wide variety of biological macromolecules with the goal of correlating these properties to the structure and function of the macromolecules. The emphasis is on molecular size and shape and the thermodynamics of molecular interactions. Analytical ultracentrifugation and mathematical modeling are the principal research techniques used. Extensive studies on the binding of plasminogen by fibrinogen have been completed. It has been shown that each fibrinogen molecule can bind four molecules of either the native glu-plasminogen or lys-plasminogen, a modified form produced by the plasmin or urokinase removal of a peptide with a molecular weight of 4,000. The equilibrium constants for the binding of either of these two forms are virtually identical. It has been demonstrated that there is very marked cooperativity of binding for the third and fourth plasminogens bound, but the possibility exists that the binding of the second plasminogen may not involve such marked cooperativity. The net change in free energy for the binding of the four plasminogens is -33.5 kcal. per mole of fibrinogen, indicating substantial hydrophobic interaction and probably significant structural alteration with binding. Calculations indicate that at physiological concentrations of these proteins, approximately one fibrinogen molecule in sixteen will have plasminogen bound to it and that molecule will most probably have four molecules of plasminogen bound. Studies are currently in progress on the association of plasminogen with plasmin inhibitor and on the association of plasminogen with the D and E fragments of fibrinogen. Studies have been initiated on the associative properties of the alpha subunit of tryptpphane synthase. This protein, with a molecular weight of approximately 29,000, can be split by limited trypsin proteolysis into two well characterized fragments with molecular weights of 20,000 and 9,000 respectively. It has been demonstrated that both of these fragments undergo reversible monomer-tetramer self-association. Studies are being undertaken to investigate the possible reassociation of the two fragments.